Many fewer small satellite galaxies are seen around the Milky Way and
M31 than the number of small dark matter halos seen
in semianalytic
models [51]
and high-resolution simulations
[52,
53]
of such systems.
But for CDM the
discrepancy arises only for satellites smaller than the LMC and SMC
[52],
and such small satellites are expected to form stars very inefficiently
[54,
55].
Semianalytic
models taking both reionization of the universe and local feedback
from supernovae into account appear to be in good agreement with
observations of the relative numbers of faint and bright galaxies in
environments ranging from the local group to clusters
[55]. This is
encouraging, but it is important to check whether such models and
high-resolution simulations
[56]
are capable of accounting for more
detailed properties of low-mass galaxies, such as their radial
distribution, metallicity and ages
[57],
and the "fundamental line" of dwarf galaxy properties
[58].

"Millilensing" by DM halo substructure may be required to account
for anomalous flux ratios in radio lensing of quasars by galaxies
producing multiple images
[59].
A concern is whether there enough halo
substructure in the inner ~ 10 kpc of galaxies, where it appears
to be needed to account for such lensing, although a recent analysis
suggests that substructure along the line of sight to these galaxies
can help account for the observations
[60]. Spectroscopic
observations can determine the mass of the lensing perturber by
comparing the magnification of different regions, for example whether
the tiny broad line region nearest the AGN is lensed, or also the much
larger narrow line region
[61].
Lensing of AGN jets can also be a useful diagnostic for substructure
[62].
Further observations are needed.